Fixed Sprite Scaling Issues

When `SpriteMaterial.useScreenCoordinates = false`, sprites of scale ( 1, 1, 1 ) now render the same size as a `PlaneGeometry` of size ( 1, 1 ) having the same location. Previously, as in [this fiddle](http://jsfiddle.net/sE6Ja/), sprite size was essentially arbitrary.

When `SpriteMaterial.useScreenCoordinates = true`, sprites of scale ( 200, 100, 1 ) render 200px by 100px, as before.

There is now no distortion when a non-uniformly-scaled sprite is rotated, as it is in [this fiddle](http://jsfiddle.net/sE6Ja/1).

`SpiteMaterial.scaleByViewport` was removed, since it was no longer needed.

Note: I was not able to test this PR on a device with `devicePixelRatio` > 1. I believe the code is correct, however.